1) Field of the Invention
The present invention relates to an NOx removal apparatus for the removal of NOx in an exhaust gas, and more particularly to a technique for control of an NH3injection rate (quantity) in an NOx removal apparatus in which the correlation between a molar ratio of an NH3 injection rate into an NOx removal apparatus with respect to a flow rate of NOx at an inlet of the NOx removal apparatus and an NOx concentration at an outlet of the NOx removal apparatus produces a V-shaped curve having a downwardly protrusive (arcuate) configuration on a plane in which a molar ratio and an NOx concentration are expressed as X-Y coordinates.
2) Description of the Related Art
At an exhaust passage of a combustion facility (gas turbine or the like) such as a thermal power plant, an NOx removal apparatus is provided with a view to removing NOx in an exhaust gas. The NOx removal apparatus is designed to reduce the NOx concentration in an exhaust gas by carrying out the reaction of NH3 to NOx on a catalyst (produced with an NOx removal catalyst, an NH3 decomposition catalyst, and others) for decomposing the NOx, together with the NH3, into oxygen, water and nitrogen. In the NOx removal apparatus, the removal efficiency (NOx removal efficiency) of NOx depends upon the injection rate of NH3; therefore, the control of the NH3 injection rate is essential in the operation of the NOx removal apparatus.
So far, as a common NH3 injection rate control method for use in an NOx removal apparatus, there has known a method based on a combination of feedforward control and feedback control. In the feedforward control, a molar ratio (=an NH3 injection rate/an NOx flow rate) of an NH3 injection rate to an NOx flow rate at an inlet is set in advance in accordance with a desired (target) NOx removal efficiency (=a desired NOx concentration at an outlet of an NOx removal apparatus/an NOx concentration at an inlet of the NOx removal apparatus), and a required NH3 injection rate is obtained on the basis of the product of this molar ratio and an inlet NOx flow rate (=an inlet NOx concentration×an exhaust gas flow rate), with the corresponding signal being corrected with a load change signal or the like. On the other hand, in the case of the feedback control, an NH3 injection rate set according to the feedforward control is corrected on the basis of a deviation between a desired NOx concentration at an outlet of an NOx removal apparatus and an actual NOx concentration detected. For these feedforward control and feedback control, various improved control methods have been proposed (for example, see Japanese Patent Laid-Open Nos. HEI 8-168639 and 9-38458, and Japanese Patent Laid-Open No. 2001-198438).
Meanwhile, depending upon the property of an NOx removal apparatus, the relationship between an molar ratio of an NH3 injection rate to an NOx flow rate at an inlet of the NOx removal apparatus and an NOx concentration at an outlet of the NOx removal apparatus can show a V-shaped characteristic having a downwardly protrusive configuration as shown in FIG. 6. In this case, a point at which a minimum value appears will be referred to as a minimum point. The reason that the NOx concentration increases conversely when the NH3/NOx molar ratio increases in some degree as shown in FIG. 6 is that the NOx removal is composed of not only an NOx removal catalyst creating an NOx reduction reaction mainly expressed by the following reaction formulas (a) to (c) but also an NH3 decomposition catalyst creating an NH3 decomposition reaction mainly expressed by the following reaction formulas (d) to (e).NO+NO2+2NH3→2N2+3H2O  (a)4NO+4NH3+O2→4N2+6H2O  (b)6NO2+8NH3→7N2+12H2O  (c)4NH3+3O2→2N2+6H2O  (d)4NH3+5O2→4NO+6H2O  (e)
When the characteristic of the outlet NOx concentration with respect to the NH3/NOx molar ratio in the NOx removal apparatus assumes a V-shaped characteristic as mentioned above, operating points on a characteristic curve corresponding to a desired value SV of an outlet NOx concentration appear at two points P1 and P2 so that the solutions for the NH3 injection rate corresponding to the desired value SV are two in number. On the other hand, as shown by two-dot chain lines in FIG. 6, a leakage NH3 quantity (a residual NH3 quantity at an outlet of the NOx removal apparatus) increases monotonically with an increase in the NH3/NOx molar ratio. Accordingly, in comparison between the operating points P1 and P2 providing the same desired outlet NOx concentration SV, the operation at the operating point P1 on the left side (on the side where the NH3/NOx molar ratio is smaller) with respect to the minimum point P0 suppresses the useless consumption of NH3 more than the other to reduce the running cost and reduces the load on the environment. That is, in the NOx removal apparatus having this V-shaped characteristic, the operating point P1 existing in the left side area with respect to the minimum point P0 is an optimum operating point which is capable of minimizing the NH3 consumption and of controlling the outlet NOx concentration to a prescribed value.
However, in the case of such a V-shaped characteristic curve, since the sign of the inclination of the input/output characteristic changes with respect to the minimum point P0, if the feedback control is simply implemented in accordance with the deviation between the desired NOx concentration SV and the actual NOx concentration as done in the conventional technique, the operating point diverges from the desired operating point P1, which can cause an uncontrollable condition.